THE OLDER SPOROPHVTE 



173 



of procambial cells. This is the single axial stele or commissural strand which had 

 not yet begun to develop in the plant with five leaves. The older leaf base, shown in 

 this section, has a single large vascular bundle, crescent-shaped in outline and con- 

 centric in structure, and there are also seen the two small concentric bundles belong- 

 ing to the stipules. 



All the leaf traces in the older leaves are concentric in structure, with well- 

 developed internal phloem, and thus differ from the collateral traces of the earliest 

 leaves. This concentric structure of the bundle is already well marked in the fourth 

 leaf, where, however, the protophloem is mainly developed upon the outer side. F 

 shows the arrangement of the bundles in a section somewhat further down. The 

 two strands from the oldest of the three leaves seen in the apex have joined the horns 

 of the crescentic central bundle, which still shows clearly its dual nature. In the 

 younger of the two portions of these sections the first tracheids have just appeared 

 near the free end, while in the older half a line of tracheids extends nearly through 

 the whole of the center. The mucilage ducts at this level are much larger and their 

 cells have become confluent. Close to the large mucilage canal can be seen the 



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A 

 Fig. 157. 



A. Section of an oldex stem of Danaa jenmani, showing cortical mucilage ducts, »»/, and central or 



commissural strand, c. //.leaf trace, r, root. X20. 



B. Shows a section higher up, with free leaf traces and commissural strand. 



section of the axial vascular strand, which somewhat higher up turns outward and 

 becomes fused with the inner face of the crescentic central stele. The leaf traces 

 soon become completely merged with the broad crescentic stele and their limits 

 can no longer be recognized. The foliar gap gradually closes up and the two por- 

 tions of the crescent separate at a point opposite so as to form a leal gap, nearly 

 opposite the first one; but I have seen no cases where a section of the central stele 

 appears as an unbroken ring, although it is possible that this might occur foi a 

 brief period. Much the same condition of things is seen lower down and it is clear 

 that in all cases, after the double leaf trace has been developed, one bundle fusts 

 with an older and one with a younger leaf trace. The axial bundle shows a central 

 mass of tracheary tissue, and in somewhat older stages an endodermis is pretty well 

 developed. Finally this bundle can be traced downward to where it connects with 

 the main stele, composed of the united leaf traces. The main stele gradually passes 

 down into the base of the stem, where it shows the structure already described for 

 the younger sporophyte. 



In older sporophytes the crescentic central stele becomes broken up permanendy 

 into two elongated masses when seen in section (fig. 157), but the junction of the 



